The present invention relates to a rotary head drum for a video cassette recorder (hereinafter "VCR"), more particularly to a rotary head drum using a fluid dynamic pressure bearing which improves the screen and voice quality in recording or reproducing a video signal.
In a conventional rotary head drum apparatus as shown in FIG. 1, there are two ball bearings 22, 22' which rotatably support the rotary shaft 21 driven by a motor, and are fixed to the lower drum 23 secured to the base plate B, and a flange 24 inserted in the upper portion of the shaft 21, and an upper drum 26 which is fixed to the flange 24 by a screw 27, and includes a rotary head 25, and two rotary transducers 28, 28' fixed on the upper surface of the lower drum 23, and under the lower surface of said flange 24 respectively, and a stator 29 being secured to the lower end portion of the lower drum by a screw 30, and a ring collar 31 which is fixed to the lower end portion of the shaft 21 by a screw 32, and to which a rotor 33 is fixed by a screw 34.
In the above mentioned conventional rotary head drum apparatus using the ball bearings, according to the rotating of the shaft 21 driven by the rotor 33, flange 24, upper drum 26, rotary transducer 28, and the rotary head 25 which are all fixed to the shaft 21 are rotated. Thus, the rotary head 25 can record or reproduce the video signal in or from a magnetic tape (not shown).
Because the rotating shaft 21 is fixed to the inner wheel of said ball bearings 22, 22' of which outer wheel is fixed to the upper and lower sides of the lower drum 23, the rotation of the shaft 21 rotates the balls and inner wheel of said ball bearings 22, 22', therefore, it is possible to support the rodial and axial loads of the upper drum 26, flange 24, ring collar 31, and the rotor 33 by the ball bearings 22, 22'.
However, in such a conventional head drum apparatus, even though the ball bearings 22, 22' are machined with mechanical accuracy, the rotating vibration and noise are generated from the ball bearings 22, 22', and such a rotating vibration has a bad effect upon the quility of the screen in recording or reproducing the video signal. Furthermore, the magnitude of the vibration more increases according to the dimensional grade and inserting error of the ball bearings 22, 22', and the actual life of the ball bearings 22, 22' is decreased by the wear of the inner and outer wheels and balls, resulting in deereasing the whole life of the VCR.
Additionally, a rotary head drum apparatus using fluid dynamic pressure bearing is well known, and one example of such a head drum apparatus is explained according to FIG. 2.
In the drawing, reference numeral 41 represents an upper drum, and 42 is a fixed shaft having plurality of lubricating grooves 42a, and it is well shown that a flange 44 having a flange boss 43 is rotatably attached to said fixed shaft 42, and said upper drum 41 is fixed to the flange 44 by a screw 46, and to which also a circular plate 45 having a spiral groove 45a, and a rotor 47 is fixed on the lower surface of the flange 44, and the lower portion of the shaft 42 is inserted into a lower drum 48 secured to a main base plate B, and a stator 49 is also fixed on the concave portion 48a of the lower drum 48. In the drawing, the reference numeral 51 is a rotary head fixed to the lower surface of the upper drum 41.
In such a rotary head drum using an aforementioned fluid dynamic pressure bearing, the upper drum 41 and the rotary head 51 are rotated according to the rotation of the rotor 47, thus, the rotary head 51 can record or reproduce a video signal in or from the magnetic tape. In such an actuation, the lower drum 48 fixed to the base plate B and a shaft 42 inserted into the lower drum 48 are not moved because then are all fixed.
According to the rotation of the rotor 47, the lubricant sealed between the flange boss 43 and the shaft 42 flows along the lubricating groove 42a of the shaft 42, and as a result, the fluid dynamic pressure is generated, and supports the radial load of the rotating bodies, and it flows into the spiral groove 45a of the circular plate 45 when the circular plate 45 fixed to the upper portion of the flange 44 is rotated, therefore, it is generated the fluid dynamic pressure to bias the rotating bodies upwardly. When the fluid dynamic pressure is higher than the pressure resulting from the axial load including the weight of the rotating bodies and the suctional power from the magnet of the rotor 47, so that the rotating bodies rise upwardly a few microns.
However, it is also a problem that because the constitution of the aforementioned head drum apparatus applying a fluid dynamic pressure is that the axial load of the rotating bodies and the suctional power of the rotor acts downwardly against the rising power of the fluid dynamic pressure, the rotating of the rotating bodies are not made sufficiently resulting from the offsetting of the rising power against the downwardly acting power each other. That is, the conventional constitation of the head drum has some problems to be resolved which the necessary rising power can not be attained because the downwardly acting power of the bodies counter acts with the upwardly acting power of the fluid dynamic pressure. Furthermore, the lubricant can be smoothly flown along the spiral groove 45a only when the clearance between the spiral groove 45a and the upper portion of the shaft 40 is maintained accurately, but the flowing of the lubricaut is not smoothly flown because the clearance is not maintained accurately according to the dimensional defect and machining error of the circular plate 45 and the right angled deviation of the shaft 42 to have bad effect upon the quility of the rotation, so that the machining of the clearance is to be very precise. Furthermore, the basic constitution shown in FIG. 2 is different from that shown in FIG. 1, so that the common use of the two types head drum apparatuses is not attained. Thus, to apply the latter head drum apparatus, it is necessary to change the whole manufacturing and assembling lines resulting in increasing the cost. Because the rotor 47 and the stator 49 of the latter head drum apparatus are installed between upper and lower drums 41, 48, it is not easy to change or repair the motor.